Heat oven to 350°F. Line 13 x 9 x 2-inch baking pan with foil. Coat foil with nonstick cooking spray.
In large bowl, beat bananas and butter. Beat in sugar until smooth. Beat in eggs, one at a time. Stir in vanilla, flour, salt, baking powder, walnuts and chips. Spread in pan.
Bake in 350°F oven until top is dry to the touch, about 40 minutes. Cool in pan on wire rack for 15 minutes. Remove bar from pan. Transfer bar to rack; let cool.
In small bowl, stir 1/2 cup confectioners sugar and 2 teaspoons water until smooth. Transfer to small plastic bag. In second bowl, stir 1/4 cup sugar, 1-1/2 teaspoons water and cocoa powder. Transfer to another bag. Snip one corner from each bag; drizzle both over top. Cut into 24 bars.